The invention relates to a hydraulic load control valve device and is described by way of examples with particular reference to its application on hydraulically driven and manoeuvred lifting cranes, especially vehicular lifting cranes.
These lifting cranes commonly have a crane boom that may oscillate up and down by a double acting hydraulic lift cylinder that acts between the crane boom and the framework or the support of the crane. This lift cylinder is part of a hydraulic system that comprises a hydraulic pump and a hand valve, by which the pump may be selectively connected with the one lift cylinder chamber when the crane boom is about to be raised and with the second lift cylinder chamber when the crane boom is about to be lowered. Simultaneously, in the first case the second lift cylinder chamber, and in the second case the first cylinder chamber is, via the hand valve, connected to the tank for the hydraulic fluid.
Normally the crane boom strives to move down by means of its own weight and the weight of a possible load that is suspended from the crane boom. For security reasons the hydraulic system is constructed such that it is not possible to lower the load if the hydraulic pump is not connected to the second lift cylinder chamber and via a connection controls a load control valve to open a connection from the first lift cylinder chamber to the tank. If there is no such securing arrangement a broken line between the first lift cylinder chamber and the hand valve could result in that the crane boom and a possible load suspended therein fall freely. Parallel to the load control valve lies a non-return valve that opens towards the first lift cylinder chamber, so it is possible to let the hydraulic fluid pass from the pump to this lift cylinder chamber. This type of security devices is particularly common in hydraulic systems where the crane operator may control the hand valve of the lift cylinder directly mechanically, e.g. by means of an operating handle.
An unsatisfactory problem of a securing arrangement of the described type and other conventional securing arrangements of similar type is that the efficiency of the hydraulic system gets low and results in that the system has a tendency to oscillate when lowering of a load.